The present invention relates to processes for the manufacture of coating or doctoring blades comprising a band of steel or other form-stable material and a wear-resistant coating applied onto said band along a longitudinal edge section thereof subject to wear.
Coating or doctoring blades tipped with rubbery or soft material are presently prepared only by moulding in a closed mould in which a band of steel or other form-stable material is placed and constitutes substrate for the coating. A liquid mix of components is injected at the lower end of a preheated mould until it appears at the opposite upper end. Care has to be taken to prevent introduction of air bubbles in the liquid material and no leakage from the mould must occur. A demoulding agent, generally based on silicones, is applied on the mould surfaces to prevent sticking of the cured material. Once filled, the mould is introduced into a circulated air oven at 80-110xc2x0 C. until curing has taken place so that the blades can be demoulded. This takes generally 45 to 180 minutes. After demoulding the blades are post-cured at 80-110xc2x0 C. for 12-24 hours.
This batch process is associated with several disadvantages, among which the main drawbacks are:
the process encounters low productivity;
each new blade geometry and blade length requires a new mould;
the mould manufacturing costs are high, especially for large moulds with complex profiles;
the larger the mould, the larger the oven necessary to preheat the mould and to cure the rubbery or soft material, and the higher the pressure necessary to fill the mould;
there are limitations in blade length because of difficulties in filling the mould without defects occurring, the need for longer pot-lives and lower viscosities, increasing mould weight, time to open, close and clean the mould etc.
For these and other reasons it is desirable to develop a simple and economic continuous process to produce such blades without limitations to length and geometry.
One object of the invention is to provide a continuous process for the manufacture of coating or doctoring blades provided with a wear-resistant soft or rubbery coating.
Another object of the invention is to provide such a process which will impart no limitations to blade length and geometry of the coated blade.
Still another object of the invention is to provide a continuous process which is commercially competitive and flexible to meet consumers specifications.
For these and other objects which will be clear from the following disclosure the invention provides a continuous process for the manufacture of coating or doctoring blades comprising a band of steel or other form-stable material and a wear-resistant polymer coating applied on said band along a longitudinal edge section thereof subjected to wear. The process involves the following steps:
a) providing continuous relative movement between said band and an application and treatment station;
b) continuously applying at said station a fast-curing polymer composition along said edge section;
c) allowing the applied composition to spread out so as to reach the very extreme of said edge section and then to cure to form an elastic and tacky-free coating; and, optionally
d) post-curing the coating at an increased temperature.
According to an alternative embodiment of such continuous process the following steps are involved:
a) providing continuous relative movement between a second band of double width compared to said first band and an application and treatment station;
b) continuously supplying at said station a fast-curing composition along a longitudinal central section of double width compared to said edge section;
c) allowing the applied composition to spread out to the desired width and then to cure to form an elastic and tacky-free coating and, optionally, post-curing the coating at an increased temperature; and
d) longitudinally cutting said second band along the middle of the coated central section thereof to form two tip-coated blades.
In the process according to the invention it is preferred to introduce before application step b) above a roughening step for said edge or central section to improve the adhesion of the coating.
It is also preferred for further improving the adhesion of the coating to apply a primer before application step b) above.
According to a preferred embodiment of the invention the fast-curing polymer composition has a pot-life of about 5 to about 30 sec.
Among preferred fast-curing polymers there may be mentioned those selected from polyurethanes, styrene-butadien polymers, polyolefins, nitrile rubbers, natural rubbers, polyacrylates, polychloroprene, thermoplastic elstomers, and polysiloxanes. It is particularly preferred to use as a polymer a polyurethane.
A suitable fast-curing polymer composition is a 3-component liquid polyurethane composition containing a prepolymer, a polyol and a chain extender. Such composition is continuously mixed with a catalyst solution, whereafter the mixture is applied onto the band to be coated.
The coating width is preferably from about 5 to about 40 mm and a preferred thickness is from about 1 to about 3 mm.
After curing of the coating it is preferred to subject the coating to a grinding operation to obtain the desired profile or geometry.